In aeronautics and especially in the maintenance of the aircraft in service, it is important to detect the presence of water in the structures of the aircraft. Water may be present in certain parts of the aircraft, especially in parts made of sandwich-type composite material. A “sandwich”-type material is a material having a honeycomb type cellular or alveolar structure lined on each side with a skin. These skins may be made out of impermeable material. They may be shaped so as to meet on the edges of the part thus forming an envelope around the alveolar structure. These parts are said to be boxed. For example, landing gear doors or hatches, rudders, radomes or elevators are parts that are frequently made of sandwich composite material. Now, the presence of water in these parts, especially in the intermediate zone, affects the behavior and weight of the structures. This may lead to unwanted behavior on the part of the aircraft in flight. At present, the presence of water in the structures is detected either by regular inspection in the maintenance phase or by signs of its presence (swelling of structures, condensation stains, etc.) or, in extreme cases, by the effects on the mechanical actuators caused by the increase in the weight of the structures.
In certain cases, a part may be damaged, leading to the complete changing of the part. In particular, when water is detected in a sandwich-type composite part, this part must obligatorily be removed and the damage caused by this water must be repaired. To this end, the aircraft has to be immobilized for a certain time. It is moreover necessary to immobilize the aircraft to inspect these parts, prior to any repair, in order to determine the presence or absence of water. To carry out an inspection of the aircraft, it is necessary to immobilize the aircraft for a relatively lengthy period. Now, immobilizing an aircraft is costly. Furthermore, the inspection techniques used at present are also costly and often difficult to implement. The inspection of the structures of an aircraft to detect the presence of water is done by techniques of thermography or radiography for example. Thermography necessitates the heating of the entire structure and radiography necessitates the isolating of the part to be inspected. These non-destructive inspection techniques are complex, their implementation is lengthy and they require special precautions of use.